1. Field of the Invention
The present invention relates generally to a method for graining the surface of a metal support of a lithographic plate, and more particularly to a preparatory abrading method for the surface of the metal support.
2. Description of Related Art
The surface of a metal web to be a support of a lithographic plate such as a web of aluminum or an alloy of aluminum and zinc is grained so that a fine unevenness called grains can be formed thereon to thereby improve its performance for printing.
Examples of the method for graining the surface of the web are a mechanical graining method using a brush roller (e.g., Japanese Patent Publication No. 50-40047), an electrolytic graining method in which electrolysis is performed in an electrolytic solution, and a chemical graining method in which the web is dipped in a chemical solution.
If, however, the locally-uneven web is grained mechanically, electrolytically or chemically, the uneven part cannot be grained uniformly, which deteriorates the printing performance at the uneven part.
Conventionally, the above-mentioned problem is avoided by preparatory abrasion to the surface of the web with a nonwoven fabric prior to the above-mentioned main graining; however, lateral defects and longitudinal streaks are formed on the web in the conventional preparatory abrasion.